SWLI is an optical profiling procedure by which a nominally equal path interferometer is illuminated by a broad band source (typically but not limited to white light) while one leg of the interferometer is axially moved (scanned) through the equal path condition (the condition when the optical path length of the two legs of the interferometer are equal). The interference signals are captured by an optical sensor, typically a camera during the scan. Interference fringes only occur in the neighborhood of the equal path condition, providing a signal to compute the relative height of the various image points of the surface illuminated by one leg to the corresponding image point in the other leg. All SWLI methods typically assume a constant scanning motion (i.e. constant velocity). If the scanning motion is not uniform, errors in the measured surface profile occur.
Unfortunately, it is often the case that the scanning motion in SWLI is not uniform. This can occur due to nonlinear motions of the scanning mechanism, or through vibrations that act on each interferometer leg differently.